Final assembly work on the British-designed Mars lander, Beagle 2, has begun.

The craft is being assembled at the Open University in Milton Keynes, UK.

"It's a momentous day because you're starting with the building of something you know will eventually end up on Mars," the OU's Professor Colin Pillinger told BBC News Online at the Farnborough International Air Show.

Beagle 2 will hitch a ride on the European Space Agency's Mars Express.

The spacecraft is due to arrive at the Red Planet in December 2003.

It will be dropped on to the Martian surface to investigate the red rock and soil, and to look for signs of life.

Beagle 2 must be ready by December, in order to be shipped to the Russian launch site at Baikonur in Kazakhstan.

The lander is being built in a specially constructed sterile assembly area.

"The assembly room will keep Beagle 2 free of terrestrial micro-organisms and other forms of contamination," Professor Pillinger said.

Mars race

Mars Express is the European Space Agency's first mission to explore the Red Planet, due to take off in summer 2003.

When it arrives at Mars, it will observe the planet using its seven instruments, and drop off Beagle 2.

The US space agency (Nasa) is planning to send a spacecraft and landers to the Red Planet about the same time.

Both missions aim to look for water on Mars and find evidence of life, past and present.

Colin Hicks, Director-General of the British National Space Centre, described Beagle 2 as a "landmark for UK space exploration".

"The mission to Mars is very important as it will help us to understand more about the surface and climate of Mars," he said.